They are same size 23in x 12x, the difference is the thickness:
5000 is 1.7/8in (48mm)
4000 is 1.1/4 (32mm)

My pump is 320gph (1212 l/h)
Going to 2 full gtx470 GPU blocks. 2500k CPU block. And HDD block. GPU and CPU overclocked very high.
It's also split off into second computer and goes to full GPU block and CPU block.

I'm thinking of upgrading the pump if I have to. I found a 480gph for $50 at petco. But I'm not sure about the radiator.
If 4000 is plenty enough to dissipate heat, I'll get that. I'm just concerned that the 5000 will require much stronger pump then the 480gph and that will cost more for pump and might make more noise.

I'm also putting the radiator outside my window. Well I ordered the 32mm. I think it will cool plenty and the 48 might require super strong pump.
I think without a fan it should be good enough. Car fans are no problem, I work with cars I have tons of spare's in the garage.